I've "made" a table with plywood. Plywood thickness was 15mm. Here is a model (one remark - the 30mm distance is 85mm actually, but it doesn't matter at all for now).
I've used 40mmx40mmx2mm corner braces for all the plywood sheets to get them together. My "table" is varnished with yacht varnish.

The obvious problem I didn't expected was that 15mm plywood is really thin, so my table top sags ~1-2cm up and down.

I'd like to ask you about possibility to reinforce the "table".

My thoughts for now:

decrease distance between lateral ribs from 1465mm to ~1400mm (and cut down rear rib as well);

increase thickness of all the ribs twice - I'm going to get the same 15mm plywood, remove the varnish and glue each pair of plywood sheets

I'm not sure about re-animating a table, as the word refers to instilling life in something dead, but the closest word in English is re-laminate and the text doesn't seem to support that term either. Could the desired word be "reinforce?" This fits better the questioning and avoids answers containing Harry Potter and magic wands.
– fred_dot_uAug 14 at 20:16

I wanted to mention that the quality of the plywood you're using is largely the issue here. I have an old desk made of chipboard (particleboard) which is known for being less stiff than ply and it is built similarly to this and the top doesn't bow. The corner braces you've used may also be partially to blame here, they may be flexing more than you'd imagine and a better connection (e.g. dowels) between the top and the 'legs' would improve the overall stability.
– GraphusAug 15 at 9:17

15mm is nearly 5/8s of an inch. This is pretty heavy plywood in most cases, and would not be expected to span 1.5M without deflecting from its own mass. Introducing a brace piece similar to the bracing in the back would probably go a long way to solving this. You might even be able to design a keyboard tray or drawer into this. But the key here is to introduce some support for the span that plywood is terrible at spanning near the front. Alternatively, you can use a metal box-, i- or, l-beam of some sort. 1/2
– jdvAug 19 at 19:20

I'm choosing not to close this one, as it seems pretty complete, and the problem seems well-known, at least with most plywood I'm familiar with. Unless this is that more expensive birtch laminate material, 1.5M is just going to sag...
– jdvAug 19 at 19:22

1 Answer
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I don't think that any of your proposed solutions really address the heart of the problem. The core issue here is that the material you used for the top is not rigid enough. 1/2" ply is not generally thick enough for large structures like table tops and cabinet boxes. For that you'd really want to use at least 3/4" (19mm) ply. (Typically 1/2" material is used for drawer boxes and bottoms.)

Moving the sides a few inches (60mm) closer together isn't going to make enough difference in the span length to resolve your issue, and thickening the legs or corner joints isn't what you need. You need a more rigid top.

One option would be to double up the top by laminating a second sheet of 1/2" ply to it. This might be trickier than you think, though, as you'll need some way to apply pressure to the entire surface while the glue cures. With 3/4" ply I would put screws in a grid every 5-10", but 1/2" material might be too thin to reliably screw into. Your best bet might be to stack weights (i.e. a pile of bricks) on it. Obviously any finish on the underside would need to be removed first.

Another option would be to attach bracing to the underside of the table. You could use the 1/2" material you have already, ripped into thin strips, say 1-2" wide. Glue them to the underside of your top (on edge, perpendicular to the top, not flat).

May I ask you for picture to better understand 2nd option? Especially this on edge, perpendicular to the top, not flat
– TarasovychAug 14 at 18:42

4

@Tarasovych SaSSafraS1232 is describing something similar to the inside of a torsion box. Looking up how those are built will help you understand. Essentially, if you imagine your table top as the top line of a T when viewed edge-on, then the descending line of the T is on edge, perpendicular to the top, not flat.
– Charlie KilianAug 14 at 20:17

Good points throughout, just wanted to mention for the metrically challenged 1/2" = 12.7mm, rounded to 13mm. 15mm ply is made so I don't think that's a typo, and it's a little beefier than you're imagining... although in this case unlikely of good quality given how much it's bowing o_O
– GraphusAug 15 at 9:09

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@Tarasovych, I would have been recommending adding a rib (or two) as advised at the bottom of the Answer, this was the most obvious way of adding stiffness to this design if they would not get in the way of your legs when you sit at the table, and without using a lot of additional material. But doubling the thickness of the top would work great and it's something to consider if you don't mind the extra cost.
– GraphusAug 15 at 9:13

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@Tarasovych it's ply. The fibres of the layers in each sheet are perpendicular to the neighbouring sheets anyway (for basic plywood; good quality thick stuff may use more angles). Any benefit from making the two adjacent layers perpendicular when you glue it up would be tiny.
– Chris HAug 15 at 14:21